The present invention relates to a variable-gain amplifying circuit and, more particularly, to a variable-gain amplifying circuit which is suitable for variably amplifying a high-frequency signal such as a video signal over a wide band range and with high accuracy.
Previous variable-gain amplifying circuits of this kind, have been disclosed in detail with regard to a multiplier in "Analog Integrated Circuit", pp. 231 to 252, written by Alan B. Grebene, translated by Shuji Nakazawa et al., and published by Kindai Kagaku Sha. In short, this multiplier has a function to output the product of two input signals. In other words, the multiplier is a circuit, in which the following equation holds if its output is designated at Z and if its two inputs are designated at X and Y: EQU Z=KXY (1)
Here, letter K designates a constant. When the multiplier satisfying the equation (1) is used as the variable-gain amplifying circuit, however, the gain is varied by the signal Y if the input X, for example, is a video input signal.
FIG. 8 is a circuit diagram showing one specific example of the aforementioned multiplier. The multiplier shown in FIG. 8 uses transistors Q.sub.1 to Q.sub.3, of which: the transistors Q.sub.1 and Q.sub.2 have both their emitters connected with the collector of the transistor Q.sub.3 ; the transistor Q.sub.3 has its emitter earthed to the ground through a resistor R.sub.E ; the transistors Q.sub.1 and Q.sub.2 have their respective collectors connected with a power supply V.sub.cc through resistors R.sub.L1 and R.sub.L2 (R.sub.L1 =R.sub.L2 =R.sub.L), respectively; an input signal V.sub.i is applied to the respective bases of the transistors Q.sub.1 and Q.sub.2 ; and an input signal V.sub.2 is applied between the base of the transistor Q.sub.3 and the ground, whereby an output signal V.sub.o is extracted from both the collectors of the transistors Q.sub.1 and Q.sub.2.
The multiplier thus constructed is frequently used because it is suitable for integration. Hence, the multiplier satisfies the following equation, provided that a relation of V.sub.T &gt;V.sub.i holds for a thermal voltage V.sub.T and that a relation of V.sub.2 .apprxeq.I.sub.E R.sub.E holds for the input voltage V.sub.2 if the current to flow through the emitter of the transistor Q.sub.3 is designated at I.sub.E : EQU V.sub.o =(R.sub.L /V.sub.T R.sub.E)V.sub.i V.sub.2 ( 2)
It can be understood that the equation (2) becomes similar to the equation (1) because the value (R.sub.L /V.sub.T R.sub.E) becomes constant. Here, a variable-gain amplifying circuit is attained if a video signal is inputted as the input signal V.sub.i to the multiplier shown in FIG. 8 and if the input signal V.sub.2 is inputted as a control signal for gain variation.
However, the variable-gain amplifying circuit described above has problems that it is very highly influenced by temperatures and that its variation change is highly limited, because it makes use of the base-emitter exponential characteristics of the transistors Q.sub.1 and Q.sub.2.
FIG. 9 is a circuit diagram showing a specific example of the variable-gain amplifying circuit which has been proposed so as to solve the above-specified problems.
The variable-gain amplifying circuit shown in FIG. 9 is constructed, as follows. In FIG. 9, reference characters Q.sub.11 to Q.sub.18 indicate transistors, and characters I.sub.1 and I.sub.2 indicate current sources which are connected between the emitters of the transistors Q.sub.11 and Q.sub.12, and Q.sub.13 and Q.sub.14 and a power supply V.sub.EE. The transistors Q.sub.11 and Q.sub.12 have both their emitters connected through a resistor R.sub.x, both their bases fed with an input signal V.sub.x and both their collectors connected through diodes D.sub.1 and D.sub.2 with a resistor R.sub.M connected in series with a power supply V.sub.cc so that an output is extracted from both their collectors. The transistors Q.sub.13 and Q.sub.14 have both their emitters connected through a resistor R.sub.y and both their bases fed with an input signal V.sub.y. The transistor Q.sub.13 has its collector connected with the emitters of the transistors Q.sub.16 and Q.sub.18, whereas the transistor Q.sub.14 has its collector connected with the emitters of the transistors Q.sub.15 and Q.sub.17. The transistors Q.sub.15 and Q.sub.16 have their bases connected with the collector of the transistor Q.sub.11, whereas the transistors Q.sub.17 and Q.sub.18 have their bases connected with the collector of the transistor Q.sub.12. The transistors Q.sub.16 and Q.sub. 17 have their collectors connected through a resistor R.sub.L1 with the power supply V.sub.cc, whereas the transistors Q.sub.15 and Q.sub.18 have their collectors connected through a resistor R.sub.L2 with the power supply V.sub.cc so that an output signal V.sub.o is obtained from between the collectors of the transistors Q.sub.16 and Q.sub.17 and the collectors of the transistors Q.sub.15 and Q.sub.18.
According to the variable-gain amplifying circuit thus constructed, it is known that the output signal V.sub.o is expressed by the following equation: EQU V.sub.o =K.sub.x V.sub.y ( 3)
(wherein the letter K designates a full-scale factor which is given by K=2R.sub.L /I.sub.1 R.sub.x R.sub.y).
However, this variable-gain amplifying circuit is composed of a large number of elements, as can be understood from FIG. 9, and has an amplified band width of about 3 MHz so that it cannot be used with high-frequency waves.